Walker, 21, had been held back the first four days of camp after the right-hander arrived with some slight soreness in his throwing arm. That delay raised concern among fans, particularly in the wake of a finger injury to Hisashi Iwakuma that will sideline the Japanese standout for at least another month.
But the Mariners were just being extra cautious with their No. 1 prospect after he had thrown a little too much prior to camp, and the sixth-ranked prospect in baseball by MLB.com erased any remaining angst by firing 25 pitches without a hitch in his first mound session of Spring Training.
"It felt great. Really good," Walker said. "I was just excited. I wanted to keep throwing."
While most bullpen sessions go 10 minutes, the youngster threw only about five minutes and then called it good, but pitching coach Rick Waits said that was exactly what the club was looking for in his first outing.
"We're not speeding him up. We're not slowing him down," Waits said. "He's been feeling great, and it was time to get him on the mound. We wanted to get him out throwing at 200 feet first, and he's done that the last 4-5 days. He's been throwing with Felix every day. He feels great. I was real pleased. I had a smile on my face today."
Walker threw only fastballs under the watchful eyes of trainer Rick Griffin and the Mariners coaches, but he will now get the thumbs up to proceed on a normal buildup.
"I'll throw another bullpen on Thursday, then I'm getting after it," he said. "This was just to see how it felt, and it felt good, so they said I'm free to go."
Walker figures to be one of the most-watched prospects in baseball this season after going 1-0 with a 3.60 ERA in three late-season starts last year.
New manager Lloyd McClendon is just getting a feel for the youngster from Yucaipa, Calif., and likes what he's seen so far.
"He's an interesting kid," McClendon said. "We all know he's very talented from a physical standpoint. I'm close to this kid. I really like him because he likes his mother and I think that speaks volumes. He really, really looks after his mom, and that tells you a lot about a young man. He's very intelligent and obviously motivated to be the best he can be. I think he's got a very bright future."
Walker, Hernandez and Yoervis Medina were the only pitchers to throw bullpens on Monday, as the other 32 pitchers in camp had already worked one or two sessions in the first four days and skipped the day in anticipation of live batting-practice sessions that will begin now when the full-squad workouts get underway.
Hernandez typically starts later than most pitchers due to his heavy workload during the season, and Medina prefers a longer buildup before taking the mound as well.
Hernandez also pronounced his opening session a success, after starting a little high with his pitches. He threw the full 10 minutes and said he's on his normal spring routine, with his first Cactus League appearance likely to come in about the seventh game of spring.
If all goes well, that would line Hernandez up for his normal Opening Day start on March 31 at Anaheim, which would be the seventh Opening Day start of his career. He's currently tied with Randy Johnson for the most Opening Day starts in franchise history.
Hernandez gave Walker some tips on how to keep his changeup down, but the Mariners' ace said he doesn't need to do much coaching with Walker or fellow rookie James Paxton. Asked if both have a chance to make the opening rotation, Hernandez left little doubt.
"They will," he said. "They have a lot of talent. The advice is just to keep working hard and do your thing. That's all they have to do."
Waits, who has been the club's Minor League pitching coordinator the past three years, sounded just as certain when asked if Walker and Paxton are prepared to compete at the Major League level.
"Yes," he said. "I don't even need to elaborate. I think they're ready. Will they leave with us? We'll see. I mean, that's up to Lloyd. But mentally and preparedness? Yes, I think they are. Our Minor League coaches have done a great job with them."